Data centre owners and managers are acutely aware of the need for better energy efficiency. With responsibility for around 1% of global electricity consumption, the sector is heavily affected by turbulence in energy costs, so there is a very strong demand for energy efficiency.
…continued from Part 1
Humidity
IT equipment can also be adversely affected by humidity – low levels increase the risk of static electricity, so spray or evaporative humidifiers may be necessary. However, high levels of humidity are also to be avoided because they can result in condensation and the corrosion of metallic equipment.
In cooler climates it may be possible to utilise colder outside air to cool data centre equipment in a process known as economisation. The absolute water content of this outside air may be low, and since the relative humidity of air decreases when it is heated, the humidity level may fall below acceptable levels unless appropriate controls are in place.
Different types of humidity measurement are required, depending on the location and application. For example, relative humidity and dew point can be measured in rooms, spaces, ducts and outdoors, whereas wet bulb temperatures would be necessary in cooling towers and evaporative humidifiers, and enthalpy sensors may be required for airside economisers. Enthalpy expresses the total heat energy (sensible heat and latent heat) involved with thermodynamic changes. These calculated humidity parameters are typically available directly from advanced humidity sensors, such as Vaisala’s.
The three most common sensor configurations for airside economiser control are dry bulb, single enthalpy, and dual enthalpy. While dry bulb control is the simplest method, it may miss potential energy savings by not opening the economiser when the ambient temperature is slightly warmer but relatively dry.
Dual enthalpy works in a similar way, except that two enthalpy sensors are employed; one monitoring outdoor air, and the other return air. The economiser will run if the outdoor air enthalpy is less than the return enthalpy.
Vaisala’s transmitters are designed specifically for applications such as air conditioning and economisation; measuring humidity and temperature, with derived outputs for dew point, wet bulb temperature and enthalpy. Temperature measurements do not generally suffer from drift, but traditional humidity sensors do, so Vaisala’s humidity sensors employ HUMICAP® technology which delivers long-term stability and insensitivity to interferences such as dust and condensation. These thin-film capacitive humidity sensors have become the industry standard in a wide variety of applications where long-term accurate, reliable, maintenance-free humidity measurements are required.
Air flow in ducts allows sensors to respond quickly to changing conditions, whereas air movement in rooms and other spaces can be slow, so some data centres prefer to use dew point temperature as a humidity control parameter because it does not depend on the temperature of the sensor.
Other sensors that may be deployed at data centres measure differential pressure in ducts and between hot and cold aisles, as well as additional outdoor meteorological parameters such as air pressure, rainfall, wind speed and direction. These measurements can be undertaken by an automatic weather station, by individual sensors, or by one of Vaisala’s WXT530 instruments, which utilise solid state technologies to minimise operating and maintenance costs. Outdoor sensors should be placed in a location with free airflow, away from any surfaces that might radiate heat and disturb measurements. Naturally, outdoor measurements must be as accurate and reliable as indoor measurements, so Vaisala’s world-leading meteorological instruments provide the long-term reliability that is required in even the harshest environments.